Metal oxide varistor (MOV), a solid component made of polycrystalline semiconductor ceramic, is currently widely used in power lightning protection devices as an overvoltage suppressor component. Normally only slight leakage current passes through the solid component, but when MOV fails to work, it can receive a large power frequency fault current from a power source and catch fire. In order to prevent from catching fire, in a technical solution using a surge protection device (SPD) equipped with a MOV, a mechanical thermal protection mechanism is employed. Owing to good thermal coupling between the thermal protection mechanism and the MOV, once the thermal protection mechanism recognizes that the temperature of the MOV has risen to a certain value because of the heat generated by the MOV, leading to melting or softening of the low-fusing alloy within the thermal protection mechanism, the thermal protection mechanism takes actions to form a gap between the MOV electrode and the thermal protection mechanism, so the MOV power supply circuit is cut off, and the MOV temperature would no longer rise. Notwithstanding, in case the current been cut off is relative large, an electric arc is probably generated in the gap. If the electric arc could not be extinguished in time, the power frequency fault current continues passing through, and the problem that MOV fires can still not be solved.
Currently, expensive computers and electronic devices used in relation to computers with large-scale integrated circuits or ultra-large scale integrated circuits are widely used in a great number of electronic equipments, and the overvoltage tolerance of these electronic equipments is considerably poor, so a surge protection device has to be employed. Generally, the surge protection device is connected between the phase line and the ground line or the neutral line. During operating the MOV is kept to be applied with power supply voltage, that is to say, the MOV is on power supply state. MOV is characterized by its capability of enduring impulse current with thousands of amperes or larger and its incapability of enduring power frequency current or temporary overvoltage for a long time. In case that MOV fails to work (i.e., unavailable), the resistance thereof is decreased, and the MOV gets a power frequency fault current from the power source, which power frequency fault current is probably tens or thousands times larger than the normal leakage current. When the power frequency fault current flows through the MOV, the MOV is easy to be damaged and failed, and probably the temperature of the MOV is increased rapidly leading to catch fire.
For purpose of preventing failed MOV from catching fire owing to a power frequency fault current, in recent years, almost all manufacturers for the surge protection device around the world employs thermal protection measure in the surge protection device design. For instance, U.S. Pat. No. 6,430,019 to Martenson, et al., titled “Circuit Protective Device” therein disclosed a surge suppression device. Referring to FIG. 1 therein, the structure of the surge suppression device is modularized, with a MOV 112 and a thermal switch component 152 set therein, and a movable non-conductive shielding plate 188 is designed. When soldered joint 182 is disconnected, the shielding plate 188 is pushed into the disconnected gap between the thermal switch component and the MOV. However, the shielding plate is inserted into the gap with vertical motion, without the function of blowing the electric arc.